This invention relates to a PWM or pulse width modulated DC motor operating system of the type in which an electronic chopper control generates a pulse signal of predetermined frequency and duty cycle to control a solid state switch in the motor armature circuit and thus control the average armature current. It further relates to such a system in which a freewheeling diode is connected across the motor armature terminals.
Such systems are known to generate high frequency transients as the electronic switch turns on and off. This is due to the characteristics of the freewheeling diode, which is switched back and forth between its forward conducting state and its reverse biased state. During each of these transitions the diode becomes a momentary short circuit and causes a significant current and voltage spike of very short duration. These spikes are transients rich in higher harmonics in the radio frequency range, which are communicated back into the power supply and radiated from the connecting wires. On a motor vehicle, where the motor operating system might be used in under-hood systems including motors powering items such as an engine cooling fan, an electro-hydraulic power steering pump or a blower motor, this locally generated RFI may be picked up by the vehicle radio or under-hood or dashboard computer controls.